Which parts to choose for a system (multiuser, multitask, with more RAM) ?

Hi all the great people here! I was searching here and on the net, and still haven't found anything working for such requirements - please help me building a PC case for these..

Here is how this PC will be used now:
* win7 x64 Pro or Ultimate (later maybe Win8, but it's gonna be Win7 for now)
* 2-3 users at the same time (fast user switching) with the following stuff opened at the same time:
- dozens (maybe even hundreds) of applications
- Chrome+Firefox+Opera+IE browsers (with at least a few plugins in each) with hundreds (maybe even a thousand) of tabs open in each (I know it may sound weird; but it's a requirement here..), and
- several virtual machines.
Usually all of this is running at the same time. And in this situation I need that switching between applications/tabs/user accounts would access the pagefile as little as possible. I see from taskmgr that in such situations CPU is not loaded much (Flash is disabled in browsers). Also, I see that on my previous PC (Win7 32-bit 4GB RAM) in such situations memory commit is usually at least 7-8GB, and therefore there is a lot of swapping going on, until the PC becomes unstable. So this new build must have a lot of RAM - 16 GB at least.

Here's the possible rough specification of some parts:

CPU - Intel Core i5/i7 (Sandy or Ivy Bridge)

RAM - DDR3 1333MHz-1600MHz 16GB dual channel (it would be ideal to have an ability to change/expand it to 32GB later on, etc.) - Kingston, Corsair or Crucial

Graphics - currently is not required to be powerful. (It would be sufficient to have a somewhat similar powerfulness like the one I had previously - ASUS HD4870 PCIE 1GB GDDR5 2XDVI - model EAH4870/2DI/1GD5).
Maybe integrated graphics would already have such power and would be enough at the moment?

Storage: 64 GB (or 128GB) SSD (for OS system files and some larger/frequently used programs)
and + 2 TB Western Digital HDD (for everything else, there will be a lot of files used)

I need the parts to be as much compatible as possible, and seriously compatible with Microsoft/Windows (Windows Hardware Compatibility List, Windows Hardware Quality Labs tested, etc. - as much as possible)

this time I don't really need abilities of overclocking the system (I just need a reliable and stable PC)

dual monitor support (no high requirements of video performance though for the additional monitor)

Which components would give the highest bang/buck ratio for around 1000Lt (250GBP)? Of course, some of the (less important) PC parts mentioned above would have to be "cropped" to lower specs in this case.

Which components would give the highest bang/buck ratio for around 2000Lt (500GBP)?

It sounds like you have it pretty well figured out, except for the processor / motherboard.
Mid range CPU: Intel i5-3470 (since overclocking is not needed)
Higher range CPU: Intel i7-3770K (best because of the several users and virtual machines)
Highest range CPU: i7-3930K (even better, but out of your budget)

For memory, simply look for a good value 1600MHz 16GB set from a brand that you trust.

For graphics, I do recommend having dedicated graphics since several people could be doing (possibly) intensive things. Any Radeon 77xx or 78xx would do great. If you are on a budget, integrated graphics will work fine, but could be a problem if several people are watching YouTube at once.

To be clear, just in case - I meant users would switch via fast user switching with many programs left running, and they want to switch fast. These programs usually won't be using a lot of CPU, but they would be using a lot of RAM.

Thanks for a reply!
To be clear, just in case - I meant users would switch via fast user switching with many programs left running, and they want to switch fast. These programs usually won't be using a lot of CPU, but they would be using a lot of RAM.
Am I not confusing something? Looks like this SSD has a better value - price and speed (random read and the advertised sequential read/write): SSD Kingston V300 120GB SATA3, 450/450MBs, IOPS 55k, 7mm, Model: SV300S37A/120G

Ah, I understand. In that case, probably any i5 would be more than sufficient for the processor.

And yep, that would be a good SSD too (better value). I only recommended the Crucial M4 because I have used several of them with very good consistent results.

... there will probably be another ~ 500 Lt (~ 125GBP) for the remaining parts of this PC case

Total: ~ 2400 Lt (~ 600 GBP)
Are these components worth the money in this situation? Will they be well compatible with each other and with the Windows HCL?

That is a good point - H61 is technically compatible but only if the manufacturer has given it a bios update, so that could possibly cause trouble.

My only recommendation then on the parts you listed is the motherboard. This AsRock B75 will certainly be compatible and is a better value for the features you get. Otherwise, I really like what you selected, and they will be compatible together. Hopefully we'll get someone else's opinion here, but I think it is good!

--I'm now also trying to pick a config with 64GB* RAM (one cheap config and one good config). Other components would be minimal, no need for high CPU speed, nor video performance. There should be an ability to connect 2 monitors (or even 3-4 monitors would be great too). Just for multiple document viewing, in fact for viewing hundreds/thousands of documents and browser windows (with Flash disabled), with a few users logged on, without any swapping/paging. Any suggestions?

*I've tested - chrome browser can eat up as much memory as many windows/tabs you throw at it. I managed to get ~5GB virtual memory usage for chrome processes in a very short test with opening a few hundred of tabs with a same document (+there were a few more GBs for OS and other apps, so it has used ~8GB easily).